Since the discovery of human milk oligosaccharides (HMO) >60 y ago, research has faced major challenges including (i) the development of methods to identify and characterize these components, (ii) the need to use HMO fractions for functional studies because single HMO were not available, (iii) the uncertainty of the purity of HMO fractions that were often "contaminated" by remainders of lactose, proteins, or glycoproteins, and (iv) the low availability of large quantities of a single HMO for animal and human studies. In the past 10 years, there has been tremendous progress in all of these areas, particularly in the development of methods for detailed structural analysis in extremely low milk volumes. The greatest success, however, is that biotechnological means are available today to produce large amounts even of a single HMO in a purity that allows human studies to be performed in the future. In this review, we summarize the current knowledge about the metabolic aspects of HMO in infants starting with the first studies by Lundblad and co-workers in the early 1980s. After discussing newer observations in recent years, the review closes with a perspective on some important questions regarding metabolic and functional aspects of HMO.